Why the first stage matters most
Every part of a receiver adds some noise, measured as its noise figure. The Friis formula shows that each stage's noise is divided by the total gain in front of it. So a good low-noise amplifier (LNA) right at the antenna sets the noise figure of the whole system, and the coax and receiver after it hardly matter. Put the same LNA in the shack, after 10 m of coax, and the coax loss is added straight on to the noise figure. Compare the first two presets to see the difference.
When it matters
Noise figure matters most where the background noise from the sky is low: on 2m and up, for satellites, moonbounce and weak-signal work, and for weather satellites and radio astronomy. On HF, atmospheric and man-made noise is far stronger than any receiver's noise, so a receiver noise figure of 10 to 15 dB is fine and a preamp rarely helps.
Noise floor and sensitivity
The noise floor is the thermal noise in your bandwidth, −174 dBm per hertz at room temperature, plus your noise figure. A narrower filter lowers it, which is why CW and FT8 get through when voice doesn't. Add the signal-to-noise ratio a mode needs to find the weakest signal you can copy. The S-meter converter turns that into an S-reading and the link budget calculator tells you how strong a signal to expect.
Questions
What is the Friis formula for noise figure?
F = F1 + (F2 − 1)/G1 + (F3 − 1)/(G1 G2) + …, with noise factors and gains as linear ratios. This calculator does it for you in dB.
Does coax loss add to the noise figure?
Yes. Before the first amplifier, every dB of loss adds a dB of noise figure. After a high-gain LNA it has very little effect.
What is a good noise figure?
Below 1 dB for a 2m or 70cm LNA and below 0.5 dB for microwave LNAs. HF receivers are typically 10 to 15 dB, which is fine because band noise is much higher.
How do I convert noise figure to noise temperature?
T = 290 × (10^(NF/10) − 1) kelvin. A 1 dB noise figure is about 75 K.